Earth Sense: Mammatus clouds and hail are signs of severe weather

Among the video footage of the Moore, Okla., tornado of May 20 available online now, some scenes shot while the twisters were touching down are disturbing. You see cars on the road and traffic patterns that look quite normal, except for the huge funnel approaching in the background.

No doubt people were hoping to make it home ahead of the tornado. But the better course of action is to seek shelter immediately in the most solid structure nearby.

There were no tornadoes but severe thunderstorms moving through our area June 13. Though there weren’t vehicles and decks flying through the air, taking cover was a good idea because falling trees and limbs present enough hazard by themselves.

Parts of Habersham and Banks counties were lucky enough to only get grazed by the heavy downpours and wind gusts. Looking south from the tall ridge on which Baldwin is located, it was possibly to see a sky filled with an eery-looking warning sign of severe storms: mammatus clouds. They resemble hundreds of bags of white flour, hanging from a low ceiling in the sky.

By themselves, mammatus clouds don’t produce severe weather, and they aren’t always an indicator that such a condition is approaching. But the process that makes them form is common in violent thunderstorm cells. Mammatus clouds form when groups of ice crystals sink downward from the cloud base because the surrounding air is warmer. Their appearance ahead of a storm system suggests that it’s time to go to a safe place, away from tree limbs and loose objects.

Another messenger of dangerous weather is hail. Even though it consists of ice falling from the sky, it’s rarely seen in the winter. Hail occurs when ice crystals repeatedly get lifted to the top of the cloud, with a new layer of ice freezing onto them every time. Eventually, they get heavy enough to fall to the ground.

Such strong activity is common during our severe weather season in the spring, and in heavy summer thunderstorms. If a tornado watch has been called for the area, spotting hail means that chances of a tornado in the next few seconds are very high. Even though the National Weather service tries to issue warnings in the range of 12 to 20 minutes, the appearance of hail tells you that there are only a few seconds left to get to safety.

Rudi Kiefer, Ph.D., is a professor of physical science and director of sustainability at Brenau University. His column appears Sundays and at gainesvilletimes.com.